It will be appreciated that speed measuring sensors are used in various installations to monitor the rotational motion of shafts, axles and wheels, particularly, vehicles provided with anti-skid braking devices.
Such angular velocity measuring sensors primarily consist of a fixed electromagnetic stator and a movable rotor member which may be provided with openings, teeth or slots. The magnetic field generated by the electromagnet is repeatedly interrupted by the openings, teeth, or slots of the moving rotor which may be fixedly attached to the rotating wheel of the vehicle. Thus, the toothed rotor causes a periodic change in the magnetic flux which induces an electrical output signal in the stator winding. The frequency of the output signal is proportional to the angular velocity of the rotating wheel.
In order to obtain the strongest and most precise output signal possible, it is necessary that the dimension of the space or air gap between the pole shoe of the magnetic stator and the teeth of the rotor be very small and be held constant. If the width of the air gap changes during operation, spurious signals, i.e., erroneous information, will adversely affect the braking force control on the wheel in question in the vehicle anti-skid system. It will be appreciated that a change in the air gap can be caused in a speed sensor for wheeled vehicles by wear and play in the axle bearing or by vibrations, particularly during a braking operation. When the rotor approaches the stator, the amount of flux cutting the coil of the stator becomes greater, and hence, the electromagnetic field building up between the rotor and stator becomes stronger. Conversely, when the rotor moves or shifts away from the pole face of the stator, the electromagnetic field coupling the coil therefore becomes weaker.
A particular critical condition occurs when a fluctuation or change in the air gap is caused by mechanical vibrations of the rotor when the vehicle is stopped and the face of the pole shoe of the stator is situated opposite a magnetizable sector of the rotor. It has been found that a nonexistent rotational motion or false movement of the vehicle wheel is simulated by this condition.
Previous attempts have been made to eliminate such unwanted interference signals or perturbing voltages from occurring when the air gap varies. Various ways and means have been employed to prevent the occurrence of such perturbations or noise voltages. This was accomplished in prior art measuring sensing arrangements by employing a signal filtering network generally consisting of attenuating elements forming part of the electronic logic circuitry which is connected in series with the speed measuring sensor. An alternate method for handling such spurious noise or interference was to design the electromagnetic stator of the speed measuring sensor with three magnetic arms or legs. The two external arms of the E-shaped stator carry the signal windings while no winding is wound on the center leg or arm. Due to this special design of the stator structure, the changes in the magnetic reluctance caused in each case in the stator arms by rotation of the rotor member are shifted out of phase by about 180.degree. relative to each other. Conversely, any variation in the magnetic reluctance caused in the magnetic arms or legs by fluctuations in the width of the air gap are essentially in phase so that the output signals generated in the windings or coils is approximately zero in the case of vibrational changes.
It will be appreciated that these previous known measuring sensors are relatively complex in design and very expensive to manufacture.